Laser diode bar stacks are generally used in a wide variety of industrial and research applications. Pluralities of laser diode bars are mounted up on a substrate to provide the multiplied power of numerous bars, versus the effect offered by a single bar. Various techniques are known for interconnecting the individual laser diode bars. For example, U.S. Pat. Nos. 5,040,187 and 5,284,790 disclose a substrate with a plurality of spaced apart parallel rectangular grooves. A continuous metallization pattern can be formed from one wall side of one groove to the other wall side of the next groove. The width of the grooves can be selected to be slightly smaller than the width of the individual laser diode bar. The substrate can be flexed to spread out the grooves apart to enable the laser diode bars to be inserted therein. When the substrate returns to a normal position, the laser diode bars are firmly compressed within the grooves to provide a secure electrical connection between the electrodes on the laser diode bars and the metallization layers on the substrate. Unfortunately, when packaged such an arrangement may cause tensile stress on the laser diode bars which can cause damage, and have to be used with soft solder. Also the methods and configurations of the disclosures are not conducive to laser diode bars alignment, as the bars tend to tip and roll within the grooves during assembly.
U.S. Pat. No. 5,128,951 to Karpinski also shows a particular type of laser diode array and method of fabrication. The disclosure has to do with providing an inexpensive mode of manufacturing a laser diode bar array. A substrate can be provided with two layers, an upper conductive layer immediately above and in flush contact with a lower non-conductive layer. The grooves for receiving the diode bars are cut into the substrate so as to completely pierce the upper layer. The disclosure purports thereby to provide a means for mounting laser diode bars which promotes conductivity between bars while also providing heat transfer into the lower electrically insulation layer. However, the maximized alignment of the bars in the grooves can be not described.
In order to avoid the disadvantage above on the laser diode bars, alternate methods for electrically interconnecting the laser diode bars have been developed. An example of such an interconnection method can be illustrated in U.S. Pat. No. 5,305,344. It discloses that laser diode submounts, rather than the laser diode bars, are inserted into the grooves on the top surface of a monolithic substrate. The submount made of Tungsten-Copper, consisted of a laser diode bar and Al2O3 mesa. The laser diode bar can be positioned in the front of the submount and connected to the top metalized surface of the mesa by a gold foil. The mesa can be located in the middle of the submount. A substrate can be provided with a plurality of spaced apart generally parallel grooves. A soft solder layer can be disposed in each of the grooves. The rear sides of the submounts are inserted into the grooves. Electrical connection between the laser diode bars can be through the copper spacers between the bottom of the substrate and the top surface of the mesas. Unfortunately, there can be still a high probability of a interconnection failure, in a mode such as degradation of the laser diode bar, solder creep onto the bar, or contamination of the laser diode bars. Other known failure modes may include alloying, melting, vaporization, and arcing, which can lead to a catastrophic destruction of the laser diode bars forming the array.
U.S. Pat. Nos. 6,636,538 and 7,060,515, as well as U.S. Pat. No. 5,898,211 disclose the different methods to build the laser diode bars array to solve the aforementioned problem by providing a laser diode package that includes a heat sink, a laser diode bar, and an electrically nonconductive substrate. Each individual package can be tested to ensure that it will function with the operational parameters desired for a laser bar array. The exposed second side surface of the laser diode bar preferably includes a layer of solder so that two packages can be joined. Numerous individual packages can be made integral in such a fashion after reflowing at the special solder melting temperature, resulting in a multi-bar laser diode array. U.S. Pat. No. 6,424,667 discloses a similar method, but using CTE matched spacers material sandwiched bonding the laser diode bar first, and then affix a substrate of electrically insulating but thermally conducting material, forming a laser subassembly. The multi-subassembly later affixed together and bond to a big heat sink to form a laser diode array. This invention greatly improves the resistance to cycling fatigue in laser-diode stacks. However, all these methods need many different solders (melting points) for multi-reflowing procedures, to make the assembly cumbersome and inefficient. Also the bar pitch of the laser diode stack has the limitation to be small enough to meet some applications.
U.S. Pat. Nos. 6,295,307 and 6,352,873 disclose a method for assembling a diode laser assembly by locating a first conductive spacer on the first conductive spacer on a planer working surface, disposing a first solder perform on the first conductive spacer, placing a diode bar on the first solder preform, disposing a second solder preform on the diode bar, placing a second conductor spacer on the second solder preform, compressing the spacer, preforms and diode bar parallel together, heating the solder preforms above their melting temperatures, and allowing the melted solder preforms to harden by cooling. This sandwiched assembly can be extended to multi laser diode bars and spacers assemblies. The spacer-bar assemblies are then attached to the electrically non-conductive heat spreader substrate, and the substrate may then be attached to the coolant manifold. The mounting surface of the heat spreader substrate may be metalized or coated with a conductive material and grooves cut through the conductive layer. The spacers are soldered or otherwise conductively connected to the conductive layer, while the diodes are situated “above” spaced apart from the grooves. The coolant manifold, heat spreader, spacers-diode bars are soldered together for good thermal conduction. U.S. Pat. No. 7,864,825 provides an improved laser diode bars assembly method which overcomes aforementioned, and other, disadvantages of the prior art. A metallization insulating layer can be soldered to a heat sink first. A plurality of parallel grooves are cut through the insulating layer or into the heat sink leaving the insulating layer in the form of a mesh with plurality of parallel streets formed on the heat sink. A laser diode stack consisting of a plurality of laser diode bars and spacers preferably, but not necessarily, formed of the same material as the heat sink can be then soldered to the grooved insulating layer with the laser diode bars located over the grooves with the spacers affixed to the streets. With this arrangement, the individual street can be free to move with the expansion of the heat sink. However, these assemblies always go through multiple steps of thermal reflowing with different melting-points solders. High possibilities of degradation of the laser diode performances, even the laser bar crack occurs from the stress induced due to any mismatch of the thermal expansion coefficients among the heat sinks, insulating layers, and the laser diode bars if the high temperature hard solders used.
U.S. Pat. Nos. 6,913,108 and 6,310,900 disclose a method for the laser diode bars array assembly. The laser diode bars, heat sinks for sandwiching bars, spacer elements for supporting bars alignment and BeO substrate are built altogether to reflow over the solder Indium melting temperature for bonding all the components into the laser diode assembly at one time. The BeO substrate includes a plurality of locating channels and coated with a metallization layer on both its upper and lower surfaces prior to assembly. The upper surface of the pedestals can be subjected to a process whereby that metallization layer can be removed therefrom. This ensures that there can be electrical isolation below the laser diode bars. Although only one solder Indium involved in the reflow procedure seems simple and efficient, a lot of problems, such as solder creeping and emitting surface contamination, greatly affect the yield of the assembly. It doesn't illustrate how to hold so many components in positions and alignment during thermal cycling. Every foregoing U.S. patent is hereby incorporated by reference in its entirety.
Thus, there is a need for a semiconductor laser diode array that can be fabricated in such a manner to eliminate known failure modes associated with fluxed soldering interconnection methods while at the same time minimizing stress caused by packaging to prevent damage to the laser diode arrays during assembly.
The present invention provides a simple and inexpensive laser diode stack assembly method, which minimizes any stress during the reflowing thermal cycling. The performance and reliability of the laser diode bar stack are greatly improved.